This invention relates generally to improvements in controlling the Czochralski process for growing silicon crystals and, particularly, to a vision system and method for measuring parameters of the silicon crystals and the silicon crystal growth process for use in controlling the growth process based on the measured parameters.
Most processes for fabricating semiconductor electronic components use monocrystalline, or single crystal, silicon as a starting material. Crystal pulling machines employing the Czochralski process produce the majority of single crystal silicon. Briefly described, the Czochralski process involves melting a charge of high-purity polycrystalline silicon, or polysilicon, in a quartz crucible which is located in a specifically designed furnace. After the silicon in the crucible is melted, a crystal lifting mechanism lowers a seed crystal into contact with the molten silicon. When the seed begins to melt, the mechanism pulls a growing crystal from the silicon melt by slowly withdrawing the seed from the melt.
After formation of a crystal neck, the process enlarges the diameter of the growing crystal by decreasing the pulling rate and/or the melt temperature until a desired diameter is reached. By controlling the pull rate and the melt temperature while compensating for the decreasing melt level, the main body of the crystal is grown so that it has an approximately constant diameter (i.e., it is generally cylindrical). Near the end of the growth process but before the crucible is emptied of molten silicon, the process gradually reduces the crystal diameter to form an end cone. Typically, the end cone is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt. During the growth process, the crucible rotates the melt in one direction and the crystal lifting mechanism rotates its pulling cable or shaft, the seed and the crystal in an opposite direction.
Although the conventional Czochralski growth process has been satisfactory for growing single crystal silicon useful in a wide variety of applications, further improvements in the quality of the semiconductor material are desirable. For example, the Czochralski process is controlled in part as a function of the diameter of the crystal being grown. Thus, an accurate and reliable system for measuring crystal diameter during the different phases of crystal growth is needed to ensure crystal quality.
Commonly assigned U.S. Pat. Nos. 5,665,159 and 5,653,799, the entire disclosures of which are incorporated herein by reference, describe a system and method, respectively, for accurately and reliably measuring crystal diameter for use in controlling the growth process of single crystal silicon. Advantageously, the system and method of these patents accurately determine the growing crystal's diameter by processing images of the crystal-melt interface generated by a camera.
Further improvements, however, are still desired. For example, the detection of false targets compromises the ability to measure the crystal diameter. Improvement is also desired to compensate for the different possible camera locations. Likewise, a redundant system is desired to reduce the possibility of a loss of diameter sensing.
In addition, hot zone apparatus is often disposed within the crucible for thermal and/or gas flow management purposes. For example, a heat shield is sometimes used to form a partial thermal cavity in the crucible for conserving heat lost from the free melt surface at the liquid-gas-solid interface. Unfortunately, hot zone apparatus can interfere with accurate and reliable diameter measurements by obscuring the camera's view of the growing crystal.
For these reasons, an improved system and method for the measurement and control of crystal diameter in the Czochralski process is desired.